In the process of drilling underground boreholes, one of the important factors affecting the success of the job is the time spent steering the well in the right direction and landing properly. Frequent changes in trajectory lead to increased hole tortuosity that increases the force required to run in and out of the hole, and also increases the total distance that needs to be drilled to get to the same target.
Drilling using a wireline cable from the bottom-hole drilling assembly (BHA) to the surface offers many benefits in terms of reduction of cost-of-drilling, and reduction of assets and personnel on location. However, with these comes a reduction in the available power available for drilling. An example of such a system can be found in one described in WO 2004072437 A (SERVICES PETROLIERS SCHLUMBERGER ET AL) Aug. 26, 2004. Such systems typically have separate drive systems for axial drive (thrust, WOB) and rotation of drill bit.
This decrease in power creates the need to optimize the drilling process by applying a lower-than-conventional force and torque at the bit, and also being able to control the rate of penetration (ROP) or advancement in real time.
Conventional drilling mainly employs two steering mechanisms; surface adjustable motor housings and rotary steerable assemblies (see, for example, U.S. Pat. No. 6,092,610 (SCHLUMBERGER TECHNOLOGY CORPORATION) Jul. 25, 2000, but neither are considered as a good match for a low power non-rotating tool. A surface adjustable housing requires multiple trips, increasing total time spent on a well and increasing tortuosity. Rotary steerable tools rely on the tool rotating for the steering mechanism.
The present invention aims to provide a drilling system that can control the direction of drilling when used with a non-rotating conveyance such as a wireline cable or coiled tubing. In the context of this invention, a non-rotating conveyance is one which cannot be used to transmit rotation along the well to a downhole drilling assembly.